Vessel trim tabs usually consist of one, usually two, independently adjustable trim planes mounted on the transom of a vessel. They are designed to provide lift in order to compensate for changes in speed, weight distribution, and water conditions. When deflected downward, the water force on the trim tab creates upward pressure, raising the stern. The surface area of the tab, the angle of deflection and the speed of the vessel all contribute to greater stem lift. When properly adjusted the trim tabs allow a moving vessel hull to be oriented generally parallel with the water surface reducing hull resistance. Trim tabs can be especially useful on planing-type vessels.
However, in some situations, it is more desirable for the stern of a vessel to sit lower in the water when in motion, usually referred to as “stern drag.” For example, when pulling a water skier behind a vessel, it is easier to pull the skier to a standing position if the stern of the boat is below the water line during the initial start-up.
Standard trim tabs are designed to rotate from zero to approximately 45° relative to the stern of a vessel. The rotation of the trim tab adjusts the stern of a vessel upward to plane the hull as necessary. But, standard trim tabs are not designed to lower the stern of a vessel.